In a typical cooling system, such as the cooling system of an internal combustion engine, a coolant reservoir is used to receive coolant overflow from the radiator of the cooling system when the coolant in the radiator expands under high temperature, and to supply coolant to the radiator when the coolant in the radiator contracts when coolant temperature is reduced.
A coolant reservoir typically includes two volumes. The first volume is the coolant volume, which is the volume occupied by the coolant contained in the reservoir. The second volume is the air expansion volume, which is the volume not occupied by the coolant and filled with air. In a conventional coolant reservoir, the air expansion volume is above the coolant volume because air is lighter than coolant. When the coolant reservoir receives coolant from the radiator, the coolant volume in the coolant reservoir increases, and the air expansion volume decreases. The reverse takes place, when the coolant in the radiator contracts and the coolant in the coolant reservoir is supplied to the radiator.
Several criteria are considered in the design and installation of a coolant reservoir. One of the criteria is that the coolant reservoir should be positioned sufficiently high relative to the radiator so that the coolant level in the reservoir is higher than the radiator coolant level. This ensures that the coolant in the coolant reservoir tends to flow into the radiator under gravity to ensure that the radiator is sufficiently filled with coolant. Another criterion is that the air expansion volume of a reservoir should be sufficiently large so that it can accommodate the amount of coolant flowing into the reservoir from the radiator.
Under certain circumstances, it is impossible for a conventional coolant reservoir to satisfy both of the above criteria. For example, when the radiator is positioned close to the roof (or cover) of the engine compartment, there may not be much space between the radiator coolant level and the roof. As a result, it is impossible to satisfy both of the above criteria, i.e., to ensure that the coolant level in the reservoir is higher than the radiator coolant level and to provide the coolant reservoir with a sufficiently large air expansion volume.